1. Field of the Invention
This invention relates to turbine apparatus and, in particular, to rotor discs having a surface thereon to facilitate detection of internal material faults occurring within the disc.
2. Description of the Prior Art
An axial flow elastic fluid turbine apparatus has rotatably disposed therein a rotor member which carries a plurality of arrays of rotating blades. The rotating blades interact with motive fluid directed thereon by stationary blades disposed within a cylindrical casing to transform the energy carried by the motive fluid into rotational mechanical energy. The rotating blades may be mounted onto the shaft portion of the rotor by insertion of a serrated root on each blade into a correspondingly shaped groove disposed on the periphery of the shaft. However, in very large, low pressure turbines, it is common practice to dispose the rotating blades in grooves fabricated on the periphery of a disc member which has been shrunk fit on to the shaft. The disc is a substantially annular shaped member forged from suitable material which, when heated to a predetermined temperature, expands so that it may be placed on the segment of the shaft which has a diameter larger than the disc bore. As the disc cools, it is firmly secured to the shaft. The disc is then said to have been shrunk fit to the shaft. One or more arrays of rotating blades may be received by each disc. In order to prevent rotation of the disc relative to the shaft, a key member is inserted into a keyway defined by aligned channels in the disc and the shaft. In prior art, both rectangular and cylindrical keys have been utilized for this purpose.
The discs, however, have been known to fracture due to stresses imposed thereon by the operation of the turbine apparatus. Fracturing of the disc can damage not only the turbine in which it is mounted, but also adjacent units, and may require shutdown of an entire power generating facility. One of the most notable happenings of this sort occurred at Hinkley Point Power Station in England in 1969. The catastrophic failure of a turbine within the Hinkley Point Station is documented and discussed in depth in articles by D. Kalderon and J. L. Gray, submitted to the Institution of Mechanical Engineers. The Kalderon paper is found in the Proceedings of the Institution of Mechanical Engineers, Volume 186, No. 31/72, while Gray's paper appears in Volume 186, No. 32/72.
Kalderon concludes that the cause of the disc burst at the Hinkley Point centered upon stress-corrosion cracking of a semicircular keyway machined into the bore of the rotor disc. The cracking was demonstrated to have occured in a plane within the disc extending longitudinally through the keyway.
It is apparent that it is of great importance to be able to determine whether stresses imposed by operation of the turbine apparatus on the disc result in the formation and propagation of cracks within the disc. Of course, visual inspection of the keyway of the fabricated rotor disc-shaft arrangement is not possible. However, as is pointed out in a comment to the above cited articles of Kalderon and Gray, found at page D121-122 of the Proceedings of the Institution of Mechanical Engineers, a two-probe ultrasonic technique has been developed and used in the prior art. The author of the comment also mentioned the fact that a single probe technique has been developed for ultrasonic interrogation for disc geometries incompatible with the two-probe technique.